This invention is directed to a method of detecting low levels of plastic deformation in metal alloys. In particular, the invention is directed to a method of detecting low levels of plastic deformation in nickel base alloys and stainless steels.
Permanent deformation will take place in metals or alloys whenever they are stressed beyond their elastic limit. In addition, metals stressed beyond their elastic limit will be imparted with a residual stress which may be detrimental, inconsequential, or beneficial depending upon the eventual use of the metal. In the situation where the residual stress would be detrimental, it is of great importance to determine (1) if the stress is present and (2) if present, the location of the stress. In the case of metal alloys, particularly nickel base alloys, residual stress may significantly increase the rate of attack of the metal subject to stress corrosion cracking conditions.
The detection of low levels of plastic deformation is difficult in large structures even with the aid of special detection devices. For this reason, it is the normal practice in the industry to stress relieve these structures whenever engineering analysis indicates the mere probability of significant plastic deformation. While stress relieving these structures compensates for the detrimental effects of residual stress, it remains a costly and time consuming procedure. Accordingly, the development of a simple and effective method of determining low levels of plastic deformation would be of great benefit because it would limit stress relieving procedures to only those structures which actually have experienced significant plastic deformation. Until the development of the method of the present invention there was no simple or effective microscopic technique for determining low levels of plastic deformation.